In operation, tungsten-halogen lamps normally contain a non-reactive gas filling such as neon, nitrogen, argon, krypton or xenon or combination thereof together with iodine, bromine, chlorine or fluorine vapor which combines with the evaporated tungsten escaping from the incandescent filament. An equilibrium concentration is attained by the gaseous species within the lamp between the temperature limits defined by the incandescent filament and coldest spot in the lamp envelope. The cold spot temperature must be sufficiently high to prevent any tungsten halide from condensing, and providing that this condition is met a continuous tungsten transport cycle operates which keeps the envelope free from tungsten. The minimum envelope temperature depends upon the halogen or halogens taking part in the cycle.
Hardglasses, such as borosilicate or aluminosilicate glass, have been successfully used for the envelope in certain generally low-wattage, tungsten-halogen lamps. However, as the lamp wattage is increased or the size of the lamp envelope is decreased, the increased wall temperature causes an increase in the rate of diffusion of the alkaline ions of the hardglass (i.e., barium, strontium and calcium ions) to the inner surface of the glass where they are able to react with the halogen gas. The result is a permanent condensation of the thus reacted halogen gas on the inner walls of the lamp, which reduces the available halogen in the lamp to a level where the tungsten/halogen cycle no longer operates, causing the lamp to blacken. After the onset of blackening, the wall temperature of the blackened portion of the bulb wall will increase, causing a more rapid diffusion, and further blackening in a "runaway" type reaction. These high temperature reactions have often limited the use of hardglass in tungsten-halogen lamps where the glass will be subjected to high temperatures. "FT-IR Diagnostics of Tungsten-Halogen Lamps: Role of Halogen Concentration, Phosphorus, Wall Material, and Burning Environment", (1991), by Laurence Bigio et al, shows that for a tungsten-halogen capsule burning in a Parabolic Aluminized Reflector (PAR) lamp with a "hot spot" temperature of 600.degree. C., the level of hydrogen bromide available in the gas phase decreases with burning time in a hardglass tungsten-halogen lamp, whereas the level of hydrogen bromide available in the gas phase remained at or above its initial levels in a quartz tungsten-halogen lamp.
It is undesirable to manufacture the lamps using excess halogen to compensate for the halogen which may react during the life of the lamp. This is because the excess halogen will react with the cooler portions of the filament and the lead wires over time, which will cause short life in lamps with long rated life, for example, greater than 150 hours.
The problem of excess activity is even more pronounced in lamps with fine wire filaments, for example, 50 watt, 120 volt filaments, since these thinner filaments have smaller cross sections and will not withstand halogen attack for very long before they fail.
In view of the limitations of using hardglass for the envelope of a tungsten-halogen incandescent lamp, the envelope of such lamps is often made from vitreous fused silica (i.e., quartz) or a high silica content glass such as one composed of ninety-six per cent silica and sold under the trademark Vycor. However, quartz and ninety-six per cent silica glass are difficult to process and require special sealing techniques to introduce the lead wires into the lamps because of their low coefficients of expansion, and thus leave something to be desired from an economic standpoint.
To prevent the reaction of the halogen constituents of the filling gas with various constituents of the lamp envelope, it is well known to use special glasses and/or a protective barrier layer.
U.S. Pat. No. 4,508,991, which issued to Wurster et al on Apr. 2, 1985, teaches a halogen-cycle incandescent lamp with an envelope of a special soft glass wherein the inner surface of the bulb is depleted of alkali ions (i.e., sodium and potassium ions) to avoid a reaction between the halogen constituents of the filling gas and the alkali constituents of the lamp envelope. The vacancies thus generated in the glass lattice may be filled by replacement ions such as lithium, magnesium and calcium. In another embodiment, the soft glass envelope having its inner surface depleted of sodium and potassium ions is coated with a protective layer of a metal and/or semi-metal oxide such as silicon dioxide (SiO.sub.2), titanium dioxide (TiO.sub.2) or barium oxide (B.sub.2 O.sub.3). According to Wurster et al, reaction between the halogen constituent of the filling gas and alkali ions is avoided in prior known halogen cycle incandescent lamps because the lamp bulb was manufactured from quartz or hard glass which both contain either no or only minor proportions of alkali ions.
U.S. Pat. No. 3,496,401, which issued to Dumbaugh on Feb. 17, 1970, teaches an iodine-cycle incandescent lamp having a lamp envelope consisting essentially of an aluminosilicate glass composition containing a low level of alkali metal oxide (e.g., sodium oxide). According to the patent, no white coatings will be formed in such a hardglass envelope containing a maximum amount of 0.10% by weight of alkali and having a strain point of at least the envelope wall temperature. Upon incandescence of the lamp filament, the envelope of the iodine-containing lamp reaches an operating temperature of between 500.degree.-700.degree. C.
U.S. Pat. No. 4,256,988, which issued to Coaten et al on Mar. 17, 1981, teaches a fluorine-cycle incandescent lamp wherein the internal surface of the lamp envelope and optionally also the exposed surface of internal components of the lamp is coated with a continuous imperforate coating composed of a metal oxide such as aluminum oxide. The aluminum oxide coating prevents free fluorine from reacting with solid tungsten and the fluorides from reacting with the silica contained in the lamp envelope.
U.S. Pat. Nos. 3,900,754; 3,902,091 and 3,982,046 teach the use of glassy coatings of metal phosphates or arsenates as protective coatings for the internal surfaces of halogen-containing electric lamps, and describe a process for the formation of defect free coatings by deposition of a solution of compounds of the metal and phosphorus or arsenic, followed by evaporation of the solvent and baking of the resulting layer.
Although the above-described techniques may be effective to some degree, there is a need in the industry for alternative solutions.